1. Field of the Invention
This invention relates to nonaqueous electrolyte secondary batteries, such as lithium ion secondary batteries.
2. Description of Related Arts
In recent years, nonaqueous electrolyte secondary batteries for performing charge and discharge by using a nonaqueous electrolytic solution to transfer lithium ions between positive and negative electrodes have been used as power sources for portable electronic devices, electric power storage, and others.
In nonaqueous electrolyte secondary batteries of this type, graphite materials are widely used as negative-electrode active materials in their negative electrodes. The use of graphite materials offers the advantage that because they have a flat charge potential and charge and discharge are performed so that lithium ions are inserted into and extracted from between the graphite crystal layers in the negative-electrode active material, the production of acicular metal lithium is reduced, resulting in small volume changes in the negative-electrode active material due to charge and discharge.
Meanwhile, in recent years, higher capacity nonaqueous electrolyte secondary batteries have been demanded in order to meet multifunction and higher performance of portable electronic devices and the like. However, the use of graphite materials as negative-electrode active materials presents a problem in that the theoretical capacity of the intercalation compound, LiC6, is as small as 372 mAh/g and therefore cannot satisfactorily meet the above demand.
To cope with this, it has recently been considered to use as a high-capacity negative-electrode active material a material capable of forming an alloy with lithium ions, such as silicon, tin, or aluminum. Particularly, silicon has a very large theoretical capacity per unit mass of approximately 4200 mAh/g and therefore various studies are being conducted toward its practical use.
However, because the material capable of forming an alloy with lithium ions, such as silicon, greatly changes its volume with storage and release of lithium ions, the expansion and shrinkage of the negative-electrode active material will be great. Thus, there arises a problem in that the electronic conductivity in the negative-electrode active material will be reduced to intermittently decrease the capacity, resulting in a deteriorated charge-discharge cycle characteristic of the nonaqueous electrolyte secondary battery.
To address the above problem, JP-A-H05-286763, JP-A-2007-87956, and JP-A-2008-27897 propose to use as a negative-electrode active material a carbonaceous composite in which a material capable of forming an alloy with lithium ions, such as silicon or aluminum, is supported on the surfaces of carbon particles and the surfaces of the carbon particles are further coated with a carbon material. Also, JP-A-H05-286763, JP-A-2007-87956, and JP-A-2008-27897 describe that the carbon particles absorb volume changes of the material, such as silicon or aluminum, with storage and release of lithium ions to prevent reduction of the electronic conductivity in the negative-electrode active material and improve the charge-discharge cycle characteristic of the nonaqueous electrolyte secondary battery.
Nevertheless, also in the nonaqueous electrolyte secondary batteries described in JP-A-H05-286763, JP-A-2007-87956, and JP-A-2008-27897, the expansion and shrinkage of silicon in the composite during charging and discharging cause reduction in electronic conductivity in the active material and between the active material and the current collector and thereby deteriorate the cycle characteristic.